Microelectrical-mechanical systems (MEMS) are miniature mechanical devices intended to perform non-electronic functions such as sensing or actuation. These devices are typically built from silicon using lithographic techniques borrowed from the semiconductor industry. Some examples of these devices are silicon pressure sensors and silicon accelerometers. Other manufacturing methods have been developed such as microembossing, stamping, microinjection molding, precision machining, and the like. These are typically used to build devices from non-silicon materials such as polymer or metal, for applications where silicon is not an appropriate material, particularly in life science applications. Examples of such devices include microfluidic devices, biochips and optical devices.
In the past decade a photosensitized polymer based on the epoxy resin Epon SU-8 (available from Microchem, Inc., Newton, Mass.) has become a popular alternative to silicon for forming micro and nanostructures. When sensitized (by blending the appropriate photosensitizing agent), this material can be crosslinked in the presence of UV light, making it photopatternable. The most common use for SU-8 is to build small epoxy structures using optical lithography. The SU-8 devices benefit from lower manufacturing costs (silicon etch is avoided), large aspect ratios (10:1 is common), optical transparency, and non-silicon surfaces (better for biological applications).
Despite SU-8's popularity, it has some significant drawbacks. Among these are (1) it is brittle, prone to shattering when stressed; and (2) it has high fluorescence making it unsuitable for fluorescent imaging applications. Brittle material properties are undesirable for many applications, particularly those that require some flexure or ruggedness, such as in stents or implants. Fluorescence is also undesirable since the material will generate much background light when performing fluorescent assays on it. We have found that Epon epoxy resin 1002F can be made photosensitive in the same way as SU-8, but is neither brittle nor fluorescent.
Epon epoxy resin SU-8 is a cross-linked polymer that has 8 epoxide groups in its monomer form. When cross-linked these produce a highly three-dimensional thermoset that is very rigid and strong, but also brittle. Epon epoxy resin 1002F is also an epoxy polymer, but it contains only two epoxy groups in its monomer form. When these monomers are linked, they form two-dimensional sheets, resulting in a thermoplastic material that has lower strength and rigidity, but can tolerate greater stress before failure. Also, we have observed that the Epon epoxy resin 1002F material has lower fluorescence properties than the SU-8 resin.